South Korea said Wednesday that it is closely monitoring possible radiation traces from North Korea's latest nuclear test, as a post-test tremor occurred due to a collapse in an underground tunnel at the test site.

Unification Minister Cho Myoung-gyon told lawmakers Tuesday that North Koreans living near the Punggye-ri nuclear site could have been exposed to radiation after the detonation.

"We are watching the situation closely due to the seriousness of harm caused by possible radiation leaks," Baik Tae-hyun, spokesman at Seoul's unification ministry, told a press briefing.

North Korea conducted its sixth and most powerful nuclear test Sunday. China's earthquake administration said that there was another tremor in the North some eight minutes after the first one due to a cave-in.

South Korea's nuclear safety agency said Wednesday it has detected no traces of radioactive materials, including xenon gas, following the North's test. (Yonhap)